tweaks travis.yml to ignore the vendor directories

Also adds go 1.9 and some small improvements inspired by
the docs at https://docs.travis-ci.com/user/languages/go

.travis.yml

@@ -2,20 +2,23 @@ language: go
 
 matrix:
   include:
-    - go: 1.7.6
-    - go: 1.8.3
-    - go: tip
+    - go: 1.7.x
+    - go: 1.8.x
+    - go: 1.9.x
+    - go: master
   allow_failures:
-    - go: tip
+    - go: master
 
 before_install:
   - mkdir -p bin
   - curl -Lso bin/shellcheck https://github.com/caarlos0/shellcheck-docker/releases/download/v0.4.3/shellcheck
   - chmod +x bin/shellcheck
 script:
-  - PATH=$PATH:$PWD/bin go test -v ./...
+  - GOPKGS_NOVENDOR=$(go list ./... | grep -v /vendor/)
+  - GOFILES_NOVENDOR=$(find . -type f -name '*.go' -not -path "*/vendor/*")
+  - PATH=$PATH:$PWD/bin go test -v $GOPKGS_NOVENDOR
   - go build
-  - diff -u <(echo -n) <(gofmt -d -s .)
+  - diff -u <(echo -n) <(gofmt -d -s $GOFILES_NOVENDOR)
   - if [ -z $NOVET ]; then
-      diff -u <(echo -n) <(go tool vet . 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');
+      diff -u <(echo -n) <(go vet $GOPKGS_NOVENDOR 2>&1 | grep -vE 'ExampleCommand|bash_completions.*Fprint');
     fi